This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Fe K-edge x-ray absorption data will be measured on single crystals of hemoglobin (Hb), which is the ubiquitous di-oxygen transfer protein in mammals. Single crystal XAS data will be measured for deoxy-, met- and oxyHb, which are the reduced, fully oxidized and oxygenated states of Hb, respectively. XAS pre-edge analysis will be carried out to correlate with theoretically generated XAS spectra (using density functional theory) to determine the ground state electronic structure of oxyhemoglobin and understand the mechanism of di-oxygen binding to heme proteins. The data will be compared to solution Fe K-edge XAS data in order to compare the electronic structure changes between solution and crystalline forms of Hb. The data will be compared to those obtained from heme model complexes that are in the S=0, S=1 and S=2 states to investigate effects of spin state changes on the Fe K-edge shifts.